Albert Barnes Commentary Numbers 7:7-9

"Two wagons and four oxen he gave unto the sons of Gershon, according to their service: and four wagons and eight oxen he gave unto the sons of Merari, according unto their service, under the hand of Ithamar the son of Aaron the priest. But unto the sons of Kohath he gave none, because the service of the sanctuary belonged unto them; they bare it upon their shoulders." — Numbers 7:7-9 (ASV)

To the Gershonites, who had to transport the hangings and coverings of the tabernacle, two wagons are assigned: to the Merarites, who had the charge of the solid parts of the tabernacle, four wagons. The furniture and vessels the Kohathites were to carry on their own shoulders. Compare Numbers 3:25-26, Numbers 3:31, and Numbers 3:36-37.